The mechanism by which gamma-irradiation-induced and near UV-induced damage to DNA is repaired will be investigated. Thymine derivatives such as 5,6-dihydroxy-5,6-dihydrothymine, 5,6-dihydroxy-5,6-dihydrothymidine and 5-hydroxy-5-methylhydantoin have been synthesized in our laboratory. These are being used as markers in the chromatographic analysis of chemically oxidized and irradiated DNA and in determination whether repair N-glycosylase and/or endonuclease activity acts upon such modified DNA. Other thymine derivatives will be synthesized and used in both high presure liquid and Sephadex LH-20 chromatographic analyses. In vitro experiments will include the preparation and characterization of modified DNA for use as substrates in enzymatic assays. Cellular experiments will consist of analysis of the modified thymine content of DNA of irradiated cells and analysis of the cytoplasm and cell medium for content of released thymine derivatives. Initial experiments will be performed with HeLa cells and normal human fibroblasts. The cells of individuals with Fanconi's Anemia and Ataxia Telangectasia will be used. These cells will be obtained from pre-existing Tissue Repositories. No patient biopsies will be performed. Attempts to purify any repair enzyme activities identified will be made. The purification of human uracil N-glycosylase from human full term normal placenta will be continued.